High-temperature processing chambers are used for depositing various material layers onto semiconductor substrates. A substrate, such as a silicon wafer, is placed on a wafer support inside the reactor. Both wafer and support are heated to a desired temperature. In a typical wafer treatment step, reactant gases are passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material onto the wafer. Throughout subsequent depositions, doping, lithography, etch and other processes, these layers are made into integrated circuits, producing from tens to thousands, or even millions, of integrated devices, depending on the wafer size and the circuits' complexity.
Various process parameters are carefully controlled to ensure the high quality of the deposited layers. One such critical parameter is the wafer temperature. During CVD, for example, the deposition gases react within particular prescribed temperature ranges for deposition onto the wafer. A change in temperature can result in a change in deposition rate and an undesirable layer thickness. Accordingly, it is important to accurately control the wafer temperature to bring the wafer to the desired temperature before the treatment begins and to maintain desired temperatures throughout the process.
Currently temperature control systems modify heating lamp power in response to temperature readings from thermocouples mounted around and below the substrate. For several reasons, the thermocouple measurements give only an estimate of the actual wafer temperature. One reason is that the wafer responds much faster to changes in heating from the lamps than do the thermocouples. Whereas the wafer undergoes relatively fast radiant heating, the thermocouple depends on slower convection heating from the wafer to read wafer temperature. In the time required for the entire thermal mass at the thermocouple tip to reach a given wafer temperature, the wafer itself may have changed to a new temperature. This heating lag causes large measurement errors under dynamic conditions. In addition, thermocouple junctions can deteriorate over time, resulting in unpredictable sources of error in their measurements.
Temperature control systems for high temperature processing chambers are sometimes calibrated using offline experiments with an instrumented wafer, specifically designed for this purpose, onto which calibrated thermocouples are attached. An example of a wafer with thermocouples attached is illustrated in FIG. 2. The wafer undergoes temperature cycling in a processing chamber. A calibration model is developed by comparing temperature measurements from the wafer itself with temperatures reported from the chamber measuring system, e.g., thermocouple, pyrometer. These experiments are very intrusive to the deposition system; production must be stopped and considerable time must be taken to set up the experiment and gather the data. Furthermore, the model developed from these offline experiments can neither anticipate nor correct for changes that occur during subsequent wafer processing from, for example, deterioration of the thermocouple junction, movement of the thermocouple, and the transparency of the thermocouple's quartz envelope. It is necessary to have accurate temperature data from production wafers in order to adjust heating in the chamber in accordance with keeping the process temperature within control limits. It is not practical to shut down the reactor and perform additional offline experiments to adjust the model as a routine part of process monitoring.
Optical pyrometers, carefully positioned in the processing chamber, can determine wafer temperature directly by measuring the light radiation emitted by the wafer. Pyrometers react to temperature changes faster than do thermocouples and, therefore, do not significantly lag the wafer temperature. Of course, if direct or reflected light from the heating lamps reaches the pyrometer, light radiation emitted by the wafer may be only a part of the radiation the pyrometer receives, and temperature readings may be in error.
In several commercial systems, a pyrometer temperature measurement from the wafer is used directly as feedback to the heating control system. In order to ensure that only radiation from the wafer reaches the pyrometer, these systems must make significant design compromises, such as through shielding the pyrometer or adjusting the placement of various components. Furthermore, the relationship between wafer temperature and emissivity changes in different temperature ranges. Thus, it is easiest to use pyrometers within specific, discrete temperature ranges, and other factors must be considered when reading temperatures over a very broad range.
Accordingly, a need exists for an apparatus and method for controlling wafer temperature that avoids both the slow response time of thermocouples and the inaccuracies associated with optical pyrometers over large temperature ranges and during periods in the processing cycle when the heating lamps are operating.